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Soil test results provide clues about struggling soybeans in 2021

This article was written by Angie Peltier, UMN Extension crops educator and Naeem Kalwar, NDSU Extension soil health specialist.

An unproductive area in a Norman County, MN field

In 2021, a farmer in Norman County, Minnesota reached out to get to the bottom of why soybeans in a large area in one of his fields either never closed rows or never grew at all. The problematic area of the field was set down into the bottom of a square-sided “bowl” with a wide county road on one side and a township road set 90 degrees to the county road (Figure 1). Neighboring areas of the field had plants that appeared to be either healthy or suffering from symptoms of iron deficiency chlorosis.

Field areas with no soybeans growing and areas with healthy soybeans growing
Figure 1. The field in Norman County, MN with an area in which no soybeans were growing (background) immediately next to an area in which soybeans were growing well (foreground). The picture was take from the adjacent township road.

When I dug up several of the plants that were struggling along in the problematic field area, there were few nitrogen-fixing nodules that did not appear to be actively fixing nitrogen and several SCN cysts all on a thin taproot with very few thin, lateral roots. What was most striking however, was the complete lack of soil structure revealed when digging up the plant.

A healthy soil tends to have aggregates, or soil particles (sand, silt, clay) held together by organic matter. Space between soil aggregates allows water infiltration and gas exchange to take place. In addition, root channels or earthworm burrows further facilitate the movement of water and air through soil layers/profile. The problematic soil had neither structure nor visible aggregates and therefore not much capacity for either water infiltration or gas exchange to take place.

Soil sampling

While we suspected that the soil itself was to blame for the poor aggregation and poor soybean growth, only by sampling the soil and sending it to a lab for analysis could we begin to get some answers.  Using a large, generator-driven, hydraulic soil probe to collect cores down to 3 feet, we collected 10 soil cores to form composite samples from both the area of problematic soybeans and the adjacent area with healthy soybeans. The 0-1 ft cores were pooled together, the 1-2 ft cores together and the 2-3 ft cores together from the two field areas, resulting in six composite samples that were sent to the NDSU soils lab for analysis. Eight inch deep soil cores were collected in both field areas to form two composite samples to determine soybean cyst nematode egg counts.

Stay tuned as the soil test results are rolled out over the week 

Recent precipitation and significant ditch, creek, river and overland flooding in northwest Minnesota has pushed back the start of this year's field work yet again. Watch for additional short articles this week detailing the results of lab analysis of soil from these two field area and what they will mean for the potential of economic soybean production where no soybeans can currently grow.

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